The present invention relates to an improved process for preparing piperidines by hydrogenation of the ring of the corresponding pyridines.
Piperidines are intermediates for the preparation of pharmaceutically active compounds (see, for example, EP-A 603,887 and EP-A 350,733, particularly pages 2 and 17). Accordingly, they have to be available in a form which is as pure as possible.
Activated piperidines are usually hydrogenated to the corresponding piperidines using palladium catalysts in alcoholic solvents, such as methanol, ethanol, isopropanol, or ethylene glycol monomethyl ether (see Heterogenous Catalysis for the Synthetics Chemist, New York 1996, Chapter 17, pages 421 to 424 and EP-A 350,733, particularly pages 65 and 66). The reaction mixture is generally worked up by removing catalyst and solvent. This gives rise to piperidines that must be purified further, for example, by crystallization, distillation, or chromatography. In this manner, undesirable by-products contained in the piperidines are removed. In addition to the expense associated with such a purification, the loss of product that occurs during further purification is disadvantageous, especially since the solvent is difficult to recycle, due to the impurities contained therein.
Accordingly, there is still a need for a process for preparing piperidines that does not require further purification of the product isolated from the reaction mixture.
This invention, accordingly, provides a process for preparing piperidines comprising catalytically hydrogenating activated pyridines in the presence of a palladium catalyst and aromatic hydrocarbon solvents, wherein the palladium catalyst is palladium-on-carbon.
Suitable activated pyridines for use in the process according to the invention are, for example, those of formula (I) 
in which
R1 represents COOR3, CONH2, COxe2x80x94NHxe2x80x94COR3, or COOH groups or two adjacent R1 groups together represent a COxe2x80x94NR4xe2x80x94CO group,
R2 represents linear or branched C1-C20-alkyl,
R3 represents linear or branched C1-C6-alkyl, phenyl, or benzyl,
R4 represents hydrogen, linear or branched C1-C6-alkyl, phenyl, or benzyl,
n represents 1 or 2, and
m represents zero, 1, or 2.
If n represents 2, two identical or two different radicals R1 may be present. Likewise, if m represents 2, two identical or different radicals R2 may be present.
Preferably, R1 represents COOxe2x80x94C1-C4-alkyl or two adjacent R1 groups together represent a COxe2x80x94N(benzyl)-CO group, R2 represents C1-C4-alkyl, n represents 1 or 2, and m represents zero or 1.
If activated pyridines of the formula (I) are employed in the process according to the invention, the corresponding piperidines of formula (II) 
in which R1, R2, m, and n are as defined under formula (I), are obtained.
The palladium-on-carbon catalysts to be used according to the invention can, for example, be catalysts that comprise from 1 to 10% by weight of palladium on any carbon. Preferably, the catalysts comprise from 2 to 8% by weight of palladium. Suitable catalysts are commercially available.
It is possible to use, for example, an amount of catalyst such that from 0.5 to 30 mmol of palladium are present per mole of activated pyridine. This amount is preferably from 2 to 15 mmol.
Suitable aromatic hydrocarbons are, for example, benzene, toluene, xylenes, and other alkyl aromatics. Preference is given to toluene. Based on 1 mole of activated pyridine, it is possible to use, for example, from 50 to 5000 g of aromatic hydrocarbon (also in the form of mixtures).
The catalytic hydrogenation according to the invention can be carried out, for example, at temperatures in the range from 20 to 200xc2x0 C. Preference is given to temperatures in the range from 50 to 150xc2x0 C., particularly those in the range from 60 to 100xc2x0 C. Suitable pressures are, for example, those in the range from 1 to 200 bar. Preference is given to pressures in the range from 3 to 150 bar, particularly those in the range from 5 to 60 bar.
The simultaneous use of temperatures and pressures close to the upper limit should advantageously be avoided, since there is otherwise a risk of the solvent being co-reduced.
After the catalytic hydrogenation has been carried out, the reaction mixture that is present can be worked up, for example, by removing the catalyst (for example, by filtration) and then removing the aromatic hydrocarbon (for example, by distillation, if appropriate under reduced pressure). Both the catalyst that has been removed and the aromatic hydrocarbon that has been removed can be recycled. If appropriate, fresh catalyst and fresh aromatic hydrocarbon can be added to the recycled catalyst and the recycled aromatic hydrocarbon, respectively.
Following removal of the catalyst and the aromatic hydrocarbon, the piperidines that are prepared are generally present in purities of more than 98%. Accordingly, further purification is not necessary. As is shown in the Comparative Examples, the customary procedure, which employs alcoholic solvents, gives without further purification only piperidines of a purity of about 94%. For the further use of piperidines as intermediates for pharmaceutics, the purity that can be obtained according to the invention is of decisive importance.
It is extremely surprising that, according to the present invention, such an advantageous process has been found, since it was not to be expected from the prior-art literature that high yields and selectivities can be obtained in the heterogeneous catalysis with palladium-on-carbon catalysts in nonpolar solvents. Furthermore, it was to be expected that not only the pyridines employed but also the aromatic hydrocarbons used as solvents should be hydrogenated, resulting not only in contaminated products but also, additionally, in a loss of solvent. However, the latter is negligible.